Nuclear magnetic resonance (NMR) is a physical phenomenon involving quantum mechanical magnetic properties of atomic nuclei in the presence of an applied, external magnetic field. NMR phenomena can be observed with an NMR spectrometer and used to study molecular physics, crystalline and non-crystalline materials. In particular, nuclear spin phenomena can be used to generate a spectrum comprised of a pattern of lines representing the various spins and spin interactions.
In solid materials, the nuclear spins experience a great number of interactions that produce very broad and featureless lines. However, the interactions are time-dependent and can be averaged by physically spinning the sample (at high rotation speeds up to 70 kHz) at an inclination of the so-called magic angle (54.74°) with respect to the direction of the external magnetic field. The averaging causes the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum.
The sample in magic angle spinning experiments is typically contained in a cylindrical sample rotor, which is a container that is permanently closed at one end and sealed by a plug or insert at the other end or may be sealed with plugs at both ends. As many experiments are performed on wet samples (for example, hydrated proteins) or samples that are liquid at room temperature and frozen for experiments, it is desirable that the seal remain completely liquid-tight even after repeated thermal cycling to cryogenic temperatures. In addition, since the plug must be slid into the rotor container after the sample has been inserted, some provision must be made for allowing air trapped between the sample and the plug to escape. Otherwise the trapped air will push the plug out of the container as a result of increased internal pressure. Further it is also desirable for the seal to be easily removable without contaminating the sample.
FIG. 1 shows a conventional insert for sealing a cylindrical NMR rotor, which is sealed at end 102. A conventional rotor insert consists of a tightly-fitting cylindrical Teflon plug 106 with a vent hole 108 in the center. The plug 106 is inserted into the open end 104 of the rotor 100. A plug 110 is screwed into the vent hole 108 after the insert 106 is pushed into the rotor 100 by inserting a wrench into the drive holes 112 and the assembly is completed with a drive cap 114 which is also pushed into the open end 104 of the rotor 100. The drive cap 114 has fins 116 against which a stream of compressed air is directed to rotate the rotor 100.
It has been found that the conventional insert 106 still leaks liquid through the plugged vent hole and between the insert and rotor wall due to capillary action. Once the insert starts to leak, it becomes lubricated by the solvent and may change position in the rotor to the extent that it may occasionally swap locations with the sample and interfere with the measurement. To prevent such leakage, it is conventional practice to add between the insert 106 and the cap 114 an additional protection layer, such as glue, silicone rubber, or wax (not shown). However, this additional layer reduces the available sample volume, adds an extra step in sample packing, can contaminate the liquid sample depending on the sealant, complicates removal of the insert, and may not survive cryogenic cooling.